Review of Mesh and Weightpaint
Shoulders
The arms and shoulders look fairly good to me. When you pose the arm above the head you need to move and rotate the shoulder bone up, too. You can do this manually or add drivers to the shoulder bones so that they are adjusted automatically when you pose the arm.
Shoulder Clipping Driver Solution (In 2 Minutes!!!) by Royal Skies LLC - adds an extra bicep bone and driver
EVERY Shoulder &amp; Collar Bone Driver (In 3 Minutes!!!) - more drivers for automated posing
To smooth out deformation you can add a Corrective Smooth modifier to the body mesh.
In the screenshot, there are no extra bones and no drivers. I've just moved the shoulder bones manually.
Comparison of corrective smooth modifier on/off. No extra bones, just moved and rotated shoulder bone
Hands
You need to add twist bones to your forearms for the rotation of the hands. Else there is massive compression at the wrists as you can see in the screenshots.
Pelvis and Upper Leg Area
Your rig has no pelvis bones. When you add them the deformation around the buttocks and thighs will get better. But the model will look a bit chubby because of the compression in the pelvis/thigh area. If you don't like this you can add a driver to the thigh bone that moves it down when the leg is rotated upwards. Additionally, you can add 2 more deform helper bones that pull the &quot;flesh&quot; (mesh) away when the leg bends. Just keep in mind this requires some manual weight paint. It's easy to do but you need to backup your work before you re-bind the mesh to the armature with automatic weights else it gets overwritten. You can transfer the weights with a Data Transfer modifier.
Pelvis and bended knees
Elbows and Knees
To prevent the &quot;scissor effect&quot; add an extra knee bone. This simulates an offset of the pivot point for the shin bone. To improve it more add 2 helper bones that simulate the deformation of the muscles and fat.
See Advanced Rigging In 60 Seconds! and Knee Clipping Drivers for details.
I've done this here for the right leg/knee. For the left knee, I tested a different method. (Advanced Deformation Series Part Two Intersection Rig (Knees and Elbows). It adds an extra bone and rotation constraint to move the pivot point of the shin bone. It doesn't look that good but I might have done something wrong because of the bone rolls.
A 3rd method I found is this knee rig (Riggy Bits #1: Simulating Volume in Joints (Part 1: Theory and Example)) that has several bones to simulate the moving pivot point in 2 directions and preserve volume. It uses bendy bones that only work in Blender and not in game engines.
Waist
Just subdivided the lower spine bone to improve bending, so she can do sit-ups exercises for example.
Toes
Added a bone to each leg to bend the toes. The automatic weights might need little manual weight paint corrections to bend the toes properly.
[upper body / elbow bones
lower body / knee bones and helper bones (right side of the body)
Note about bone rolls
Please note the bone rolls of the bones. In your rig, the leg bones bend around the Z-axis. In most other rigs (Rigify and tutorial rigs) it's the X-axis. As consequence, you have to swap the X and Z-axis, and the sign (-/+) for some driver variables and bone rotation constraints.